Abstract

An ideal bioherbicide should be easy and cheap to mass produce, viable and efficacious in controlling the target weed within definite time. A better understanding of the strategies of mass producing of inoculum is important to evaluate the suitability of Exserohilum longirostratum as bioherbicide. Exserohilum longirostratum has been reported as potential bioherbicide in controlling several grassy weeds; however, its control efficacy has several shortcomings. A study was conducted to determine the suitability of E. longirostratum as bioherbicide for controlling barnyard grass (Echinochloa crus-galli (L.) Beauv. spp. crusgalli) both in the glasshouse, and in the field. In the Carbon:Nitrogen (C:N) ratio test, 10:1 (C:N) produced the highest biomass (0.75g/100mL) and, 7.5:1 and 10:1 (C:N) resulted in highest production of healthy spores (6.4 x 105 spore per mL and 6.6 x 105 spore per mL respectively) compared with control treatment without external carbon and nitrogen source (1.4 x 105 spores per mL). The later produced about 4 times less than in the 7.5:1 and 10:1 C:N ratio test. An early infection of target plant by the pathogen may assure a successful disease development. A 105 spores/mL with 30% oil and a 5 day-old (DO) mycelium culture in the ratio of 1:5 (wt:vol) with 30% oil infected high disease severity on barnyard grass (Echinochloa crus-galli (L.) Beauv. spp. crusgalli). Both inoculum forms are effective in causing severe disease on the weed. The overall disease progress rate for the conidia on barnyard grass (rLc= 2.50 unit/day) was not significantly different with disease progress rate for mycelium (rLm= 2.27 logit/day). With the presence of herbicide resistant biotype of barnyard grass, pre-disposing the barnyard grass with sublethal dose of chemical herbicides will enhance better control of the weed and are crucial in integrated weed control. Base on this study, E. longirostratum was able to grow in some of the herbicides tested at the sublethal dose. Pretilachlor and cyhalofop/buthyl supported high germination of E. longirostratum compared with other chemical herbicides at all doses and to non-chemical amended control. These chemicals also supported > 60% appressorium formation at 0.25x of the recommended rate compared with the control. Compatibility of E. longirostratum with chemical herbicide may increase fungus efficacy on weed control. Sublethal rate of herbicide combined with pathogen may lead to synergistic effect, potentially increasing weed control and reducing producing costs. Understanding the course of the infection and development of E. longirostratum aids in elucidating the mechanism of host death and in determining the suitability of E. longirostratum as the biocontrol agent on barnyard grass. Germination of E. longirostratum took place about 4 hours after inoculation on both barnyard grass (67%) compared with rice (51.25%) 24 hours after inoculation. Many of the germ tubes extended along the surface of the junction of the epidermal cells. Appressoria formation were significantly higher on barnyard grass (92.4%) compared with only 10.9% on rice. Most infection process ceased at the stage of germ tubes elongation in rice. The fungus fails to cause infection. This observation indicated that rice is not a compatible host for this fungus. This conclusion was further supported by the result of the mini plot trial. Rice plants produced more tillers and high biomass compared with the untreated counterpart. However, barnyard grass was effectively controlled by E. longistratum with or without pretilachlor added as auxiliary. High mortality of barnyard grass caused its numbers of tillers and dry weight dramatically low compared to rice. Barnyard grass reduced >90% of dry weight from the control. The result of this research indicated E. longirostratum has the potential to be used as bioherbicide for integrated management of barnyard grass in rice field.